U.S. Pat. Nos. 4,784,722 and 4,935,939 (incorporated herein by reference) disclose a thermal mass-transport process for forming lenses in III-V material or alloys thereof.
Specifically, FIGS. 5 and 6 and column 9 of the '722 patent disclose the concept of lens formation wherein a circular oxide mask is formed on a top planar surface of a suitable substrate. Material not underlying the mask is then etched away leaving a top step. A new oxide mask 77 is formed over the structure and the process is repeated until a stepped-shape approximating the desired lens surface is formed. The structure is then subjected to heat treatment to transport a volume of mass from above the desired lens contour line to fill voids below the contour line, thereby forming a spherical lens.
The height of each step is dependent upon the desired lens shape and is also made to satisfy the equal volume requirement (see Col. 4, lines 7-14 of '722 patent). That is, the volume to be relocated is made equal to the volume to be filled. See also FIG. 3 of the '939 patent which illustrates formation of a collimating lens in the center and a spherical mirror coaxial to the collimating lens (as described at Col. 2, line 65-Col. 4, line 42).
The above-described process is time-consuming and tedious and requires painstaking attention to detail which can result in errors. Specifically, multiple mask and etching steps are required and subsequent masks must be aligned on a no longer planar surface.